This invention relates to a marine engine driveshaft coupling, and more particularly to the coupling between the crankshaft and upper driveshaft end in an outboard motor or the like.
The above identified U.S. Pat. No. 3,350,879 is illustrative of an outboard motor having an engine and an elongated driveshaft, which drives a propeller disposed in the device's lower unit. U.S. Pat. No. 3,337,221 illustrates one form of connection between the crankshaft and driveshaft of an outboard motor.
In known prior constructions, the crankshaft is mounted for rotation in the engine housing and includes a splined rotor hub adapted to receive the upper splined end of an elongated relatively rigid driveshaft, forming a coupling therebetween. The driveshaft has normally been of a constant outside diameter (O.D.) throughout its length, with the O.D. of the splined portion being axially constant. The inner diameter (I.D.) of the splined portion of the crankshaft has normally been dimensioned to mate with the splined portion of the driveshaft. The respective splined portions of the assembled driveshaft and crankshaft intersected adjacent the entrance of the driveshaft into the crankshaft, that is, at or near the outer terminus of the crankshaft.
Problems have arisen at the area of intersection of the shafts, in that the coupling tended to fail in this area. In attempting to locate the cause of the problem, stress risers or wear areas were observed in one or both of the members adjacent their intersection. It was determined that the driveshaft was too rigid and that the resulting stress risers or wear areas caused fatigue of the metal parts, which ultimtely caused the coupling failure.
One attempt to solve the problem was to taper the upper end of the driveshaft, including its splined portion, but such a construction was found to be very expensive to manufacture.
It is an object of the invention to solve the problem of failure of the crankshaft-to-driveshaft coupling of a marine engine in a simple and reasonably economical manner.
In accordance with the various aspects of the invention, the splined portion of the upper end of the driveshaft is provided with means to mechanically reduce the driveshaft rigidity by providing an intermediately positioned groove or undercut therein. The depth of the undercut is contemplated as being approximately the same as the channels between the splines, and the axial extent or length of the undercut is contemplated as being approximately equal to or greater than the undercut depth. In the assembled unit, the undercut is positioned at the outer terminus of the crankshaft. The result is a coupling wherein the rigidity of the driveshaft is reduced and it is free to flex more easily at the intersection of the members so that the stress riser area is relieved and fatigue failure is substantially reduced. The construction furthermore, provides greater compliance and allows for more mis-alignment than with the known devices.